tests: Add a -V option to set new variable TEST_VAR_DIR

tests: Add a -V option to set new variable TEST_VAR_DIR

Part of preparation to be able to install the tests.  The current var/
subdirectories will end up somewhere in /usr/local/ or /usr/ and we
don't want to put temporary files there.

This creates a temporary TEST_VAR_DIR by default.

If -V is specified then the given directory will be used as
TEST_VAR_DIR.  This allows the current behaviour where individual
integration tests can be run against already-running daemons 9with
sockets and other stuff already created) to save time and trouble.

Yes, there are plans for a clean-up option...  ;-)

Signed-off-by: Martin Schwenke <martin@meltin.net>
(This used to be ctdb commit 8e4ec9a40bb7d392d7474b067a74fa121c069007)